Fuel assemblies consumed and no longer usable in a terminal stage of fuel cycle are called spent fuels. Spent fuels contain fission products and other highly radioactive substances, and must be cooled thermally, and hence they are cooled for a specific period in a cooling pit at a nuclear power plant. Then they are transferred into a shielding container called cask, and conveyed and stored at reprocessing plant or storage facility by means of truck or ship. When transferring the spent fuel assemblies into the cask, a holding element having a lattice section called basket is used. Each one of the spent fuel assemblies is put into each of the cells formed as a plurality of storing spaces in the basket, and a proper holding strength to withstand vibration during transportation is assured.
Various types of casks are disclosed in publications such as “Atomic Eye” (Nikkan Kogyo Shuppan Production, Apr. 1, 1998) and Japanese Patent Application Laid-open No. 62-242725.
FIG. 13 is a perspective view of an example of a cask. FIG. 14 is an axial direction sectional view of the cask shown in FIG. 13. A cask 500 is composed of a shell main body 501, a resin 502 which is a neutron shield provided on the outer circumference of the shell main body 501, its outer tube 503, bottom 504 and lid 505. The shell main body 501 and bottom 504 are carbon steel forged parts which are gamma-ray shielding materials. The lid 505 is composed of a primary lid 506 and a secondary lid 507 made of stainless steel or the like. The shell main body 501 and bottom 504 are bonded by butt welding. The primary lid 506 and secondary lid 507 are fixed to the shell main body 501 by means of stainless steel bolts. A metal O-ring is interposed between the lid 505 and shell main body 501, and the inside is kept airtight.
Many inner fins 508 are provided, between the shell main body 501 and outer tube 503, for heat conduction. The inner fins 508 are made of copper in order to enhance the heat conduction efficiency. The resin 502 is injected into the space formed by the inner fins 508 in fluid state, and is solidified by thermosetting reaction or the like. A basket 509 is a bundle structure of 69 square pipes 510 as shown in FIG. 14, and is inserted into a cavity 511 in the shell main body 501.
The square pipes 510 are made of aluminum alloy mixing neutron absorbing material (boron: B) so that the inserted spent fuel assemblies may not reach a critical condition. At both sides of the cask main body 512, moreover, a trunnion 513(only one is shown) for suspending the cask 500. At both ends of the cask main body 512, a buffer materials 514 assembling wood or other shock-absorbing materials are provided (only one side is shown). Reference numeral 515 is a cell for accommodating the spent fuel assemblies.
The light-water type reactor is classified into the boiling water reactor (BWR) and pressurized water reactor (PWR). This cask 500 is the cask for storing the spent fuel assemblies used in BWR. The structure of the fuel assembly for BWR is shown in FIG. 15. The fuel assembly for BWR has four channel boxes 601 arranged in lattice, and a cross-shaped control rod 608 is located in the central area, and the reactivity of fuel rods 603 is controlled by the vertical move of the control rod 608.
The fuel assembly 600 has 8×8 fuel rods 603 arranged in lattice. Each fuel rod 603 is a long bar of about 4 meters in length. A support lattice 607 is provided for supporting the fuel rod 603. In each fuel rod 603, a plurality of columnar pellets 631 formed by sintering powder of uranium oxide are inserted as fuel into a cylindrical clad pipe 633 composed of zirconium alloy, and held by a spring 632 inserted in the upper part of the clad pipe 633. A handle 610 is used for lifting up and down the fuel assembly 600. The spent fuel assembly inserted in the cell 515 in this cask 500 is a fuel assembly for BWR 600 shown in FIG. 15.
By contrast, the fuel assembly shown in FIG. 16 is used in PWR. In this fuel assembly for PWR 700, control rods 708 are properly distributed among fuel rods 703, and the fuel rods 703 and control rods 708 are arranged in a 17×17 lattice form. In this 17×17 fuel assembly 700, one measuring pipe is disposed in the center, and 24 control rods 708 are distributed.
The reactivity of the core can be controlled by moving the control rod 708 up or down. Each fuel rod 703 is a long bar of about 4 meters which is same as the fuel rod 603 for BWR, and hence a support lattice 707 for supporting the fuel rod 703 is provided. In each fuel rod 703, a plurality of pellets 731 are inserted into a cylindrical clad pipe 733 composed of zirconium alloy, and held by a spring 732 inserted in the upper part of the clad pipe 733. In this cask, 70 units of spent fuel assemblies are contained in the cell.
FIG. 17 is an axial direction sectional view of the cask for PWR. As shown in FIG. 17, a cavity 811 of a cask 800 for PWR contains a basket 809 forming a cell 815 of rectangular section by alternately combining plates 817 extending in the radial direction. Each plate 817 is composed of aluminum alloy mixing B as neutron absorbing material, same as the square pipes 510 for BWR.
However, each plate 817 has a through-hole of cooling water passage 816 extending in the axial direction, known as a water zone, and when cooling the spent fuel assemblies, each basket cell and this through-hole are filled with water to moderate neutrons, so that the neutrons may be absorbed efficiently by the plates 817 and the resin 802. The water filling the cooling water passage 816 is discharged after a specified cooling period, and the passage is dried.
This cooling water passage 816 is required because the uranium enrichment factor of fuel assembly for PWR is higher than that of the fuel assembly for BWR, the uranium charging amount per fuel assembly is larger, the sectional area of the fuel assembly is wider, and hence the reactivity is higher in the system arranging these assemblies. The distance dd shown in FIG. 17 is such a distance that it assures the subcriticality of the spent fuel assembly. The distance dd for PWR is required to be longer than the same for BWR. The cells 815 are not arranged in lattice but are deviated in position, which is intended to dispose the spent fuel assemblies for PWR, having a wider sectional area as compared with the spent fuel assemblies for BWR, efficiently in the cavity 811.
In the spent fuel assemblies for PWR mentioned above, the control rods 708 distributed among the fuel rods 703 arranged in lattice are extracted, and the spent fuel assemblies 700 from which the control rods 708 are extracted are inserted and stored in the cells 815 formed by the basket 809 in the cask 800.
Therefore, the vacancies left over by the extracted control rods 708 or measuring rod, not shown, in the spent fuel assemblies are dead spaces, and it is desired to utilize these spaces effectively.
Further, the plates or square pipes for forming the basket for PWR are required to form the cooling water passage 816, and formation of plates or square pipes is more complicated as compared with formation of plates or square pipes for BWR, and it takes much time and labor.
Besides, the sectional area of the spent fuel assemblies for PWR is rectangular and wide, and also the cooling water passage 816 is needed, and hence the accommodation efficiency of the spent fuels in the cask is poor.